1. Field of the Invention
The present invention relates to an electrophotographic toner for developing an electrostatic latent image formed by electrophotography and the like, and to an electrophotographic image forming method using the toner.
2. Discussion of the Background
Electrophotographic image forming methods typically include the following processes:
(1) the surface of an image bearing member (a photoreceptor) is charged (charging process); PA0 (2) the image bearing member (photoreceptor) is exposed to light to form an electrostatic latent image thereon (latent image forming process); PA0 (3) the latent image is developed with an electrophotographic toner to form a toner image on the image bearing member (developing process); PA0 (4) the toner image is transferred onto a receiving material (transferring process); and PA0 (5) the toner image on the receiving material is fixed upon application of heat, pressure, solvent vapor, or combination thereof to prepare a copy image (fixing process). PA0 (1) wet developing methods using a liquid developer including a fine color pigment or dye dispersed in a liquid: and PA0 (2) dry developing methods such as cascade methods, magnetic brush methods and powder cloud methods, which use a toner including a colorant such as carbon black dispersed in a binder resin. PA0 (1) good charge properties; PA0 (2) good thermal properties such as good resistance to blocking; PA0 (3) good mechanical strength; PA0 (4) good chemical properties such as resistance to chemicals and good fluidity; PA0 (5) good powder properties; PA0 (6) a property so as not to contaminate or damage a developing device, a photoreceptor and other elements in an image forming apparatus; PA0 (7) good stability so as not to deteriorate even when used for a long time; and PA0 (8) good cleaning property.
The methods for developing electrostatic latent images are broadly classified into the following methods:
Recently the dry developing methods are widely used.
The dry developers are broadly classified into one-component developers including a magnetic toner, and two-component developers including a toner and a magnetic carrier which imparts a frictional charge to the toner. Two-component developers are suitable for forming multiple color images because of having good color reproducibility.
The properties requisite for the dry toners are as follows:
With respect to the fluidity, a toner preferably has good fluidity so as to be easily fed and charged in a developing device. Therefore, a technique in which a fine particulate inorganic material having an average particle diameter of from about 5 nm to about 100 nm is added to a mother toner is disclosed. In addition, a technique in which a fine particulate inorganic material having an average particle diameter of from about 0.5 .mu.m to about 5 .mu.m is added to a mother toner to impart good cleaning property to the resultant toner has been disclosed in Japanese Laid-Open Patent Publications Nos. 57-174866, 60-136752 and 60-32060. Further, a technique in which a fine particulate organic material having an average particle diameter of from about 0.05 .mu.m to about 5 .mu.m is added to a mother toner has been disclosed in Japanese Laid-Open Patent Publications Nos. 60-186854, 60-186859, 60-186864 and 60-186866.
In such toners including a fine particulate inorganic or organic material as an external additive, a problem such that the external additive is released from the toner surface or embedded into the toner tends to occur.
When the external additive is released from or embedded into the toner, the toner loses good fluidity and good charge properties which are imparted to the toner by the external additive. Therefore, the toner causes many problems such as deterioration of image qualities.
In particular, the external additive released from a toner tends to damage the surface of a photoreceptor, resulting in formation of a black streak image.
In attempting to solve these problems, a technique such that an inorganic metal oxide having a relatively large particle diameter of from 150 nm to 5 .mu.m is added to a toner is disclosed in Japanese Patent Publication No. 2-45188. However, the technique has a drawback in that such a metal oxide has poor adhesion to the toner, and when the toner is used for a long time, the metal oxide is released from the toner. In addition, a technique such that a silica is adhered to a toner is disclosed in Japanese Laid-Open Patent Publication No. 2-167561. However, the technique has a drawback in that the additive is rather easily embedded into the toner.
In attempting to prevent an external additive from being released from a toner, Japanese Laid-Open Patent Publication No. 9-96923 discloses a technique in which two kinds of fine particulate hydrophobic metals and a toner are mixed to prepare a toner on the surface of which the fine particulate hydrophobic metal is uniformly adhered. In order to prevent the external additives from being released from the toner, the toner and the additives have to be mixed upon application of strong force in a mixer, and thereby the additive is embedded in the toner. Therefore, the technique does not produce good effects.
In addition, in attempting to prevent an external additive from being released from or embedded into a toner, Japanese Laid-Open Patent Publication No. 5-119515 discloses a toner which is prepared by adding a plate-shaped fine particulate material to a mother toner. However, the toner has a drawback in that when the toner is used for a long time, the plate-shaped fine particulate material tends to release from the toner.
Toner preferably has a good fixing property. When a toner is used in an image forming apparatus having a fixing device including a heat roller, a toner image contacts the heat roller upon application of pressure while the toner of the toner image is melted. Therefore, an offset problem in that a part or an entire part of the toner image tends to be offset to the heat roller. The offset image is then transferred to the receiving material or the following receiving material, resulting in formation of background fouling.
In attempting to solve the offset problem, Japanese Patent Publication No. 51-23354 discloses a toner using a crosslinked polymer as a binder resin, and Japanese Laid-Open Patent Publication No. 49-6523 discloses a toner including a low-molecular-weight polyolefin such as polypropylene and polyethylene. In addition, in attempting to improve the fixing property and the offset problem, Japanese Laid-Open Patent Publication No. 9-80814 discloses a toner which uses two kinds of binder resins having different molecular weights and glass transition temperatures, wherein the differences of the binder resins in molecular weight and glass transition temperature are specified.
However, these toners cannot exhibit a combination of good resistance to offset phenomenon, good resistance to abrasion, and good fixing property.
In addition, in attempting to prepare a toner having good low temperature fixability and resistance to offset phenomenon, a technique in which a polyester resin including a novolak type phenolic resin as a constitution unit is used as a binder resin. However, the toner does not exhibit a combination of good resistance to offset phenomenon and good low temperature fixing property. In addition, the olefin tends to be released from the toner and then adhered to a carrier, resulting in contamination of the carrier. Therefore, the charging properties of the toner deteriorate, resulting in formation of background fouling in the developed toner images. Further, the toner including an olefin has poor fluidity and thereby the toner tends to be unevenly fed to a developing area, resulting in formation of images having uneven image density.
In the developing method using a heat roller, the surface of the heat roller is typically formed with a releasing material such as fluorine-containing resins. In addition, an offset preventing liquid such as silicone oils is typically applied on the surface of the heat roller to cover the surface of the heat roller with the liquid. When the liquid is heated by the heat for fixing, the liquid generates a nasty odor. In addition, a mechanism for supplying the liquid is needed, and thereby the image forming apparatus becomes complex and the manufacturing costs of the apparatus increase.
In addition, in a developing device using a magnetic toner, a problem in that images fixed in low temperature conditions have poor fixing property tends to occur because the toner includes magnetic particles therein, which do not melt by the heat for fixing.
Further, magnetic particles included in a magnetic toner are typically hard, and are present in toner particles while the magnetic particles project their top from the surface of the toner. Therefore, when the magnetic toner is used for a developing method using a magnetic brush, the image bearing member tends to be damaged, in particular, under low temperature conditions, and thereby the image qualities of the produced images deteriorate.
In a latest high speed copier using a heat roller to which relatively low heat energy is supplied (i.e. in an energy-saved high speed copier), it is difficult to prevent the offset problem. Namely, in order to avoid the offset problem, the binder resin in the toner preferably has good toughness and good melt fluidity However, in order to impart the good melt fluidity to the toner, the toner must be heated to a relatively high temperature. Therefore, the toner does not meet the requirement of low temperature fixing. When a resin having a relatively low glass transition temperature and a low molecular weight is used in a toner as a binder resin to improve the low temperature fixing property, the resultant toner has poor toughness, resulting in occurrence of the offset problem.
In order to prepare a toner having toughness so as to avoid the offset problem, a polymer having an average molecular weight of not less than 100,000 is typically used. In particular, vinyl polymers are typically used for this purpose. In order to fix a toner including a high molecular weight vinyl polymer at a low temperature, the glass transition temperature of the vinyl polymer is minimized as far as the resultant toner does not cause the blocking problem. Alternatively, a method in which a plasticizer is added to a toner to improve the low temperature fixing property. However, these methods decease not only the minimum fixing temperature (i.e., the temperature below which toner images cannot be perfectly fixed), but also the offset temperature (the temperature above which the offset phenomenon occurs). Therefore, the methods only move the fixing temperature range between the minimum fixing temperature and the offset temperature (this range is also called as fusing latitude) toward a low temperature side. If the weight average molecular weight of the binder resin is increased to prevent the decrease of the offset temperature, the toner loses the low temperature fixing property obtained by decreasing the glass transition temperature or adding the plasticizer. In addition, when the weight average molecular weight of the binder resin is increased, the toner is not easily pulverized, resulting in deterioration of productivity of the toner.
Polyester reins having both a relatively low glass transition temperature and a low molecular weight can be easily manufactured. This is the difference between polyester resins and vinyl polymers. Therefore, a toner having low temperature fixability can be easily prepared by using a polyester resin as a binder resin. However, such a toner seriously causes the offset problem, and therefore cannot be used for a developing device using a heat roller.
In attempting to prepare a toner having both an advantage of polyester resins of imparting low temperature fixability and an advantage of vinyl polymers such that that the offset problem tends not occur at a relatively high temperature, a toner including a polyester resin and a vinyl polymer is disclosed in Japanese Laid-Open Patent Publication No. 54-114245. However, the compatibility of a polyester resin and a vinyl polymer is not good. In particular, the more the difference in molecular weight between a polyester resin and a vinyl polymer, the worse the compatibility thereof. When the mixture of these resins is observed with a microscope, one of the resin is present like an island in a sea of the other resin. This island-sea phenomenon can be observed when plastics having poor compatibility are blended. This island-sea phenomenon is described in detail in Plastics 13, No. 9 pp 1 (1962).
When the binder resin components of a toner achieve such an island-sea state, the other components of the toner such as a colorant and a polarity controlling agent cannot be uniformly dispersed in the toner. Therefore, when the toner is repeatedly used toner particles having an opposite charge are formed resulting in formation of background fouling in the resultant toner images.
In addition, a method is provided in which a graft polymer in which a polyester resin and a vinyl resin is grafted using a common segment is used as a binder resin. However, the resultant toner does not have a combination of the advantages of the polyester resin and the vinyl resin, but has the average property of the polyester resin and the vinyl resin.
Because of these reasons, a need exists for an electrophotographic toner having good charge properties, and producing good images without causing undesired images such as black streak images and background fouling even when repeatedly used for a long time.